1. Field of the Invention
The present invention relates to a phase conjugate laser mirror employing Brillouin-enhanced four wave mixing which allows multiple independent laser apertures to be phase locked producing an array of diffraction-limited beams with no piston phase errors.
2. Description of Related Art
Phase conjugate mirrors employing Stimulated Brillouin Scattering (SBS) have become very useful and in some cases essential in high power laser systems. These mirrors, placed at the end of an amplifier chain somewhere in the mid-range of the amplification path, reflect the light with a phase wave front that is nearly exactly the inverse of that of the incoming laser beam. The reflected light retraces its path through the amplifiers, canceling out any wave front distortions that accumulated in the forward direction. This results in near aberration free output beams that exhibit the minimum beam divergence allowed by optical diffraction, hence the description xe2x80x9cdiffraction-limited.xe2x80x9d
The mechanism responsible for the reflectivity of the SBS mirrors is the generation of an intense acoustic wave inside the SBS nonlinear material. This acoustic wave serves as a very efficient Bragg grating which reflects the incoming light. Since the acoustic grating travels at the speed of sound through the material, in the same direction as the input light, the reflected light is frequency shifted from the input light by 100 MHz to  greater than 10 GHz, depending on the SBS medium. The frequency shifted output is referred to as the Stokes wave and the frequency shift is referred to as the Stokes shift. The active material for the conjugators has most often been a liquid or high pressure gas. A stimulated Brillouin scattering (SBS) phase conjugate laser mirror which uses a solid-state nonlinear gain medium instead of the conventional liquid or high pressure gas medium is disclosed in U.S. Pat. No. 5,689,363, incorporated herein by reference.
Scaling of solid state lasers to high energy and high average power is often limited by the maximum cross sectional size of the gain medium that can still provide acceptable wavefront in the presence of strong thermal loading. To achieve more output energy, a larger gain medium volume is needed but increasing the volume in most practical manners decreases waste heat Extraction efficiency.
The present invention is a technique in which multiple beams from separate laser amplifiers can be combined into a single phase locked beam. Using standard phase conjugator materials such as liquids (CC14, fluorinert) and high pressure gases (100 atmospheres of V2 or Xe), this technique can be used for relatively long pulses of 200 ns to over 1 ms and for shorter pulses in the 1 to 200 ns range. The technique can be used for pulses in the sub-nanosecond range, provided a sufficiently fast response (broad-bandwidth) phase conjugation medium is employed. The physical dimensions of the system must be sized so that the round trip path through the combiner does not exceed the physical length of the pulse (the beam must overlap itself within the combiner). In addition to phase locking the beams in each of the individual beam paths, the technique corrects for wavefront distortion introduced by thermally-loaded amplifiers and passive errors in other optical components.
The present technique overcomes limitations, especially in temporal phase instability, that were encountered by previous inventions employing only a simple focusing SBS setup. In earlier approaches, multiple beams were focused into a single SBS cell with an attempt to overlap all of the foci. Good spatial overlap, however, is generated only over relatively small volumes. In contrast, the present invention folds the beams within the SBS cell, causing them to cross paths multiple times. This SBS phase conjugate mirror design provides for overlap between the first and third (last) foci in the SBS medium. Once the SBS process is above threshold, the four wave mixing interaction at this crossing causes Stokes shifted light to be scattered from the input beam around the optical loop and directly into the Stokes output beam. Since this establishes xe2x80x9cclosed-loopxe2x80x9d operation, the nonlinear process no longer depends on noise to sustain the reflected Stokes beam and becomes very stable for the duration of the input pulse. In the absence of the optical architecture of this invention, temporal phase instabilities were the key problem that has prevented the successful operation of an SBS phase locking mirror.
This invention has a basis-in U.S. Pat. No. 5,689,363, incorporated herein by reference, where the architecture is described for a high energy, high average power laser system employing a single frequency oscillator and a Nd:glass amplifier. The patent discusses a four-wave mixing SBS phase conjugator that reduces the threshold for SBS initiation and reliably phase conjugates a beam with a pulse duration of  greater than 1 ns. Also briefly mentioned in the patent is the idea of combining multiple beams in a single 4-wave mixing phase conjugator. The present invention provides a more robust extension to phase locking of multiple beams, the technique of using a xe2x80x9ccombxe2x80x9d mirror optical architecture and the extension of this concept to shorter laser pulses.